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7.2 KiB
HTML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /><link rel="stylesheet" type="text/css" href="libvirt.css" /><link rel="SHORTCUT ICON" href="/32favicon.png" /><title>libvirt architecture</title></head><body><div id="container"><div id="intro"><div id="adjustments"></div><div id="pageHeader"></div><div id="content2"><h1 class="style1">libvirt architecture</h1><p>Currently libvirt supports 2 kind of virtualization, and its
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internalstructure is based on a driver model which simplifies adding new
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engines:</p><ul><li><a href="#Xen">Xen hypervisor</a></li>
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<li><a href="#QEmu">QEmu and KVM based virtualization</a></li>
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<li><a href="#drivers">the driver architecture</a></li>
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</ul><h3><a name="Xen" id="Xen">Libvirt Xen support</a></h3><p>When running in a Xen environment, programs using libvirt have to
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executein "Domain 0", which is the primary Linux OS loaded on the machine.
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That OSkernel provides most if not all of the actual drivers used by the set
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ofdomains. It also runs the Xen Store, a database of informations shared by
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thehypervisor, the kernels, the drivers and the xen daemon. Xend. The xen
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daemonsupervise the control and execution of the sets of domains. The
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hypervisor,drivers, kernels and daemons communicate though a shared system
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busimplemented in the hypervisor. The figure below tries to provide a view
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ofthis environment:</p><img src="architecture.gif" alt="The Xen architecture" /><p>The library can be initialized in 2 ways depending on the level
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ofpriviledge of the embedding program. If it runs with root
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access,virConnectOpen() can be used, it will use different ways to connect
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tothe Xen infrastructure:</p><ul><li>a connection to the Xen Daemon though an HTTP RPC layer</li>
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<li>a read/write connection to the Xen Store</li>
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<li>use Xen Hypervisor calls</li>
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<li>when used as non-root libvirt connect to a proxy daemon runningas root
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and providing read-only support</li>
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</ul><p>The library will usually interact with the Xen daemon for any
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operationchanging the state of the system, but for performance and accuracy
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reasonsmay talk directly to the hypervisor when gathering state informations
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atleast when possible (i.e. when the running program using libvirt has
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rootpriviledge access).</p><p>If it runs without root access virConnectOpenReadOnly() should be used
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toconnect to initialize the library. It will then fork a libvirt_proxy
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programrunning as root and providing read_only access to the API, this is
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thenonly useful for reporting and monitoring.</p><h3><a name="QEmu" id="QEmu">Libvirt QEmu and KVM support</a></h3><p>The model for QEmu and KVM is completely similar, basically KVM isbased on
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QEmu for the process controlling a new domain, only small detailsdiffers
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between the two. In both case the libvirt API is providedby a controlling
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process forked by libvirt in the background andwhich launch and control the
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QEmu or KVM process. That program calledlibvirt_qemud talks though a specific
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protocol to the library, andconnects to the console of the QEmu process in
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order to control andreport on its status. Libvirt tries to expose all the
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emulationsmodels of QEmu, the selection is done when creating the new
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domain,by specifying the architecture and machine type targetted.</p><p>The code controlling the QEmu process is available in
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the<code>qemud/</code>subdirectory.</p><h3><a name="drivers" id="drivers">the driver based architecture</a></h3><p>As the previous section explains, libvirt can communicate using
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differentchannels with the Xen hypervisor, and is also able to use different
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kindof hypervisor. To simplify the internal design, code, easemaintainance
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and simplify the support of other virtualization engine theinternals have
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been structured as one core component, the libvirt.c moduleacting as a
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front-end for the library API and a set of hypvisor driversdefining a common
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set of routines. That way the Xen Daemon accces, the XenStore one, the
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Hypervisor hypercall are all isolated in separate C modulesimplementing at
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least a subset of the common operations defined by thedrivers present in
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driver.h. The driver architecture is used to add supportfor other
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virtualization engines and</p><ul><li>xend_internal: implements the driver functions though the
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XenDaemon.</li>
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<li>xs_internal: implements the subset of the driver availble though theXen
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Store.</li>
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<li>xen_internal: provide the implementation of the functions possible
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viadirect Xen hypervisor access.</li>
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<li>proxy_internal: provide read-only Xen access via a proxy, the proxycode
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is in the <code>proxy/</code>sub directory.</li>
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<li>xm_internal: provide support for Xen defined but not running
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domains.</li>
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<li>qemu_internal: implement the driver functions for QEmu and
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KVMvirtualization engines. It also uses a qemud/ specific daemon
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whichinterracts with the QEmu process to implement libvirt API.</li>
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<li>test: this is a test driver useful for regression tests of thefront-end
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part of libvirt.</li>
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</ul><p>Note that a given driver may only implement a subset of those
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functions,for example saving a Xen domain state to disk and restoring it is
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only possiblethough the Xen Daemon, in that case the driver entry points are
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initialized toNULL.</p><p></p></div></div><div class="linkList2"><div class="llinks2"><h3 class="links2"><span>main menu</span></h3><ul><li><a href="index.html">Home</a></li><li><a href="news.html">Releases</a></li><li><a href="intro.html">Introduction</a></li><li><a href="architecture.html">libvirt architecture</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="format.html">XML Formats</a></li><li><a href="python.html">Binding for Python</a></li><li><a href="errors.html">Handling of errors</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="html/index.html">API Menu</a></li><li><a href="examples/index.html">C code examples</a></li><li><a href="ChangeLog.html">Recent Changes</a></li></ul></div><div class="llinks2"><h3 class="links2"><span>related links</span></h3><ul><li><a href="https://www.redhat.com/archives/libvir-list/">Mail archive</a></li><li><a href="https://bugzilla.redhat.com/bugzilla/buglist.cgi?product=Fedora+Core&component=libvirt&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=MODIFIED&short_desc_type=allwordssubstr&short_desc=&long_desc_type=allwordssubstr">Open bugs</a></li><li><a href="http://virt-manager.et.redhat.com/">virt-manager</a></li><li><a href="http://search.cpan.org/~danberr/Sys-Virt-0.1.0/">Perl bindings</a></li><li><a href="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/index.html">Xen project</a></li><li><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="12" value="Search..." /><input name="submit" type="submit" value="Go" /></form></li><li><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-90x34.gif" alt="Made with Libxml2 Logo" /></a></li></ul><p class="credits">Graphics and design by <a href="mail:dfong@redhat.com">Diana Fong</a></p></div></div><div id="bottom"><p class="p1"></p></div></div></body></html>
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